Rotating and stationary hardware used in power-generation sections of gas-turbine engines require protective coatings. Highoperating temperatures, generally in excess of 1,50°F, make for severe conditions. Components often see high-temperature oxidation or hot-corrosion attack. And in certain situations both oxidation and hot corrosion work in tandem.
Engines have begun operating at dramatically higher temperatures over the past decade as a means of boosting fuel efficiency. New families of coatings and processes have evolved to help handle these more severe conditions.
As one would expect, engine-operating conditions influence coating selection. For example, flight gas-turbine engines might employ one coating if used primarily overland, another if in a sea-salt environment. Also important is the typical engine cycle. Will it be subject to long flights or to short hops.
Likewise, land-based turbines will use different coatings depending on whether the machine generates base electricity or handles peak demand periods. Additional factors coming into play include the type and purity of fuel that fires the engine.
Suppliers have developed coating processes specifically for power generation components such as blades, vanes, and seals. The special coating processes range from relatively simple pack cementation to elegant electron-beam physical-vapor-deposition (EB-PVD) techniques.
Coating developments span the gamut from diffusion aluminide to multielement nickel, cobalt, chromium, aluminum, yttrium (NiCoCrAlY), and zirconia-based ceramic compositions. As gas-turbine-engine designs continue to grow in sophistication so also do coatings and the processes used to deposit them.
Information provided by Dave Punola, business center manager for Thermatech Coating, Howmet Corp., Whitehall, Mich., (231) 894-7157.